Oxide semiconductor thin-film transistor

ABSTRACT

A thin-film transistor includes a gate electrode, a source electrode, a drain electrode, a gate insulation layer and an oxide semiconductor pattern. The source and drain electrodes include a first metal element with a first oxide formation free energy. The oxide semiconductor pattern has a first surface making contact with the gate insulation layer and a second surface making contact with the source and drain electrodes to be positioned at an opposite side of the first surface. The oxide semiconductor pattern includes an added element having a second oxide formation free energy having an absolute value greater than or equal to an absolute value of the first oxide formation free energy, wherein an amount of the added element included in a portion near the first surface is zero or smaller than an amount of the added element included in a portion near the second surface.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from and benefit of Korean PatentApplication No. 2010-47630, filed on May 20, 2010, which is herebyincorporated by reference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Exemplary embodiments of the present invention relate to an oxidesemiconductor thin-film transistor (TFT). More particularly, exemplaryembodiments of the present invention relate to an oxide semiconductorTFT capable of preventing a reaction between an oxide semiconductor andan electrode material.

2. Discussion of the Background

Generally, a display device includes a plurality of switching elements.The switching elements include a gate electrode connected to a gateline, a semiconductor pattern insulated from the gate electrode, asource electrode connected to a data line to be electrically connectedto the semiconductor pattern, and a drain electrode spaced apart fromthe source electrode. Examples of a TFT used as a switching element of adisplay device include an amorphous-silicon (a-Si) TFT, a poly-silicon(poly-Si) TFT, and an oxide semiconductor TFT.

The a-Si TFT may be uniformly formed on a large-sized substrate at lowcost; however, the a-Si TFT may have low charge mobility. In contrast,the poly-Si TFT may have a higher charge mobility and lowerdeterioration rate of its element characteristics compared with the a-SiTFT. However, a liquid crystal display (LCD) with the poly-Si TFT mayhave a more complex manufacturing process than an LCD with an a-Si TFT,which may incur a larger manufacturing cost than the manufacturing costof an LCD with an a-Si TFT.

The oxide semiconductor TFT may be manufactured using a low temperatureprocess and may be used in a large-sized display panel. In addition, theoxide semiconductor TFT may have relatively large charge mobility.However, an oxide semiconductor may react with substituent components ofa source electrode or a drain electrode such as a component metal,thereby inducing oxide semiconductor deoxidation and extraction of acation included in the oxide semiconductor.

When the oxide semiconductor is reduced with cation extraction, thecomposition of a channel layer of a TFT changes so that the chargemobility of the TFT decreases. In addition, a threshold voltagetemporally varies. Moreover, resistance values of wiring (i.e., gateline and data line resistances) may increase due to metal leaching fromthe oxide semiconductor. Thus, the TFT may not function due to metalextraction, which may lead to electrical instability and diminishreliability of the switching element.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention provide an oxidesemiconductor TFT that may prevent a reaction between an electrode orwiring material and an oxide semiconductor material.

Exemplary embodiments of the present invention also provide an oxidesemiconductor TFT that may minimize a decrease of carrier mobilitywithin an oxide semiconductor material.

Exemplary embodiments of the present invention further provide an oxidesemiconductor TFT that may have increased reliability.

Additional features of the invention will be set forth in thedescription which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention.

An exemplary embodiment of the present invention discloses an oxidesemiconductor thin-film transistor (TFT) that comprises a gate electrodedisposed on a substrate; a source electrode and a drain electrodedisposed on the gate electrode and comprising a first metal element witha first oxide formation free energy; a gate insulation layer disposed onthe gate electrode and insulating the gate electrode from the sourceelectrode and the drain electrode; and an oxide semiconductor pattern.The oxide semiconductor pattern comprises a first surface contacting thegate insulation layer; a second surface contacting the source electrodeand the drain electrode, the second surface disposed opposite the firstsurface; and an added element with a second oxide formation free energy,the absolute value of the second oxide formation free energy beinggreater than or equal to the absolute value of the first oxide formationfree energy. In the oxide semiconductor pattern, an amount of the addedelement in a portion proximate to the first surface is zero or smallerthan an amount of the added element in a portion proximate to the secondsurface.

An exemplary embodiment of the present invention also discloses an oxidesemiconductor thin-film transistor (TFT) that comprises a gate electrodedisposed on a substrate; a source electrode and a drain electrodedisposed on the gate electrode and comprising a first metal element; agate insulation layer disposed on the gate electrode and insulating thegate electrode from the source electrode and the drain electrode; and anoxide semiconductor pattern. The oxide semiconductor pattern comprisesan oxide material comprising an ion of at least one of indium (In),gallium (Ga), zinc (Zn), and tin (Sn); a first surface contacting thegate insulation layer; and a second surface contacting the sourceelectrode and the drain electrode. The second surface is disposedopposite the first surface. In the oxide semiconductor pattern, aportion proximal to the second surface further comprises Zn ion, and anamount of Zn ion in a portion proximal to the first surface is zero orsmaller than the amount of Zn ion in the portion proximal to the secondsurface.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention, andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is a plan view showing an array substrate according to anexemplary embodiment of the present invention.

FIG. 2 is a cross-sectional view taken along line I-I′ of FIG. 1.

FIG. 3 shows photographs of the decrease in the amount of metalextracted from an oxide semiconductor corresponding to an increase in anamount of an added element in an oxide semiconductor.

FIG. 4 is a graph showing the relationship between RF power duringsputtering and charge carrier mobility in an oxide semiconductor.

FIG. 5A and FIG. 5B are graphs showing a relationship between an amountof an element added to an oxide semiconductor and distance from a firstsurface of the oxide semiconductor pattern shown in FIG. 2.

FIG. 6A, FIG. 6B, FIG. 6C and FIG. 6D are graphs showing free energyassociated with metal oxide formation in the reaction of various metalswith 1 mole of an oxide material of indium (In), gallium (Ga), zinc (Zn)and tin (Sn).

FIG. 7 shows photographs of a decrease in a metal amount extracted froman oxide semiconductor that corresponds to an increase of an amount ofzinc ion included in an oxide semiconductor.

FIG. 8 is a cross-sectional view of an oxide semiconductor TFT accordingto another exemplary embodiment of the present invention.

FIG. 9 is a cross-sectional view of an oxide semiconductor TFT accordingto another exemplary embodiment of the present invention.

FIG. 10 is a cross-sectional view of an oxide semiconductor TFTaccording to another exemplary embodiment of the present invention.

FIG. 11 is a cross-sectional view of an oxide semiconductor TFTaccording to another exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The invention is described more fully hereinafter with reference to theaccompanying drawings in which embodiments of the invention are shown.This invention may, however, be embodied in many different forms andshould not be construed as limited to the embodiments set forth herein.Rather, these embodiments are provided so that this disclosure isthorough and will fully convey the scope of the invention to thoseskilled in the art. In the drawings, the size and relative sizes oflayers and regions may be exaggerated for clarity. Like referencenumerals in the drawings denote like elements.

It will be understood that when an element or layer is referred to asbeing “on,” “connected to,” or “coupled to” another element or layer, itcan be directly on, directly connected to, directly coupled to the otherelement or layer, or intervening elements or layers may be present. Incontrast, when an element is referred to as being “directly on,”“directly connected to,” or “directly coupled to” another element orlayer, there are no intervening elements or layers present.

Hereinafter, “deoxidation” refers to chemical deoxidation of an elementor dissociation of an element from oxygen such as in breaking a chemicalbond between the element and oxygen. The oxygen may be in anoxygen-containing species or unbound in the oxide semiconductordiscussed below. Further, the phrase “at least one of,” when used with alist of elements, means that the element(s) selected may be just onesingle element from the list or any combination of two or more elementsfrom the list.

FIG. 1 is a plan view showing an array substrate according to anexemplary embodiment of the present invention. FIG. 2 is across-sectional view taken along line I-I′ of FIG. 1.

Referring to FIG. 1 and FIG. 2, the array substrate includes a substrate101, a gate line GL, a data line DL, a thin-film transistor (TFT), and apixel electrode 450.

The TFT includes a gate electrode 110, a gate insulation layer 150, asemiconductor pattern 300, a source electrode 210, and a drain electrode230.

The gate electrode 110 is electrically connected to the gate line GL.For example, the gate electrode 110 may protrude from the gate line GLas shown in FIG. 1. In this case, the gate electrode 110 is integrallyformed with the gate line GL so that a boundary between the gateelectrode 110 and the gate line GL is not obvious. The gate line GL andthe gate electrode 110 may be formed from a conductive layer, i.e., agate conductive layer. The gate electrode 110 may include a metal suchas aluminum (Al), copper (Cu), molybdenum (Mo), titanium (Ti), tantalum(Ta), silicon (Si), neodymium (Nd), niobium (Nb), yttrium (Y), scandium(Sc), or a metal alloy thereof. Moreover, the gate electrode 110 mayinclude an optically transparent and electrically conductive materialsuch as indium tin oxide (ITO), indium zinc oxide (IZO), andaluminum-doped zinc oxide (AZO). However, materials of the gateelectrode 110 and the gate line GL are not limited to these enumeratedmaterials. For example, the gate electrode 110 and the gate line GL mayhave a single layer structure. In another example, the gate electrode110 and the gate line GL may have a multi-layer structure in whichplural conductive layers are deposited on the substrate 101 or acombination of plural conductive layers and plural insulation layers aredeposited on the substrate 101.

A gate insulation layer 150 covers the gate electrode 110. The gateinsulation layer 150 insulates the gate line GL from the data line DLand insulates the gate electrode 110 from the semiconductor pattern 300.

A source electrode 210 is electrically connected to the data line DL.For example, the source electrode 210 may protrude from the data line DLas shown in FIG. 1. The source electrode 210 is electrically connectedto the semiconductor pattern 300. A drain electrode 230 is spaced apartfrom the source electrode 210 and is electrically connected to thesemiconductor pattern 300. The data line DL, the source electrode 210,and the drain electrodes 230 may be formed from an identical conductivelayer, i.e., a data conductive layer.

For example, the data line DL, the source electrode 210, and the drainelectrode 230 may have a single layer structure. In another example, thedata line DL, the source electrode 210, and the drain electrode 230 mayhave a multi-layer structure in which plural conductive layers aredeposited on the substrate 101 or a combination of plural conductivelayers and plural insulation layers are deposited on the substrate 101.The source electrode 210 and the drain electrode 230 may include, e.g.,a metal such as Al, Cu, Mo, Ti, Ta, Nd, Nb, Y, Sc, or a metal alloythereof.

The array substrate may further include a passivation layer 410 coveringthe source and drain electrodes 210 and 230. The passivation layer 410may protect the TFT. The passivation layer 410 may include an inorganicinsulation material or an organic insulation material. However, amaterial of the passivation layer 410 is not limited to these materials.

The pixel electrode 450 is electrically connected to the drain electrode230. For example, the pixel electrode 450 may be disposed on thepassivation layer 410 and may be electrically connected to the drainelectrode 230 through a contact hole 415 penetrating the passivationlayer 410. The pixel electrode 450 may include an optically transparentand electrically conductive material such as ITO, IZO, and AZO. However,materials of the pixel electrode 450 are not limited to these.

The semiconductor pattern 300 is disposed on the gate insulation layer150. The semiconductor pattern 300 may be used as a channel layer of theTFT. The semiconductor pattern 300 may include an oxide material havingleast one element of indium (In), gallium (Ga), zinc (Zn) and tin (Sn).For example, the semiconductor pattern 300 may include an oxidesemiconductor material such as a zinc oxide, tin oxide, indium oxide,In—Zn oxide, In—Sn oxide, In—Ga—Zn oxide, In—Zn—Sn oxide, andIn—Ga—Zn—Sn oxide. These materials may be used alone or in variouscombinations. Alternatively, the semiconductor pattern 300 may be anoxide material that has additional elements.

The semiconductor pattern 300 may include an added element having a freeenergy for oxide formation (hereinafter, referred to as “second oxideformation free energy”) whose absolute value is greater than or equal tothe absolute value of the free energy for oxide formation (hereinafter,referred to as “first oxide formation free energy”) of a metal elementincluded in the source and drain electrodes 210 and 230. The source anddrain electrodes 210 and 230 contact the semiconductor pattern 300.Here, the oxide formation free energy denotes the free energy associatedwith forming an oxide material in the reaction of an oxide with 1 moleof a predetermined element.

Hereinafter, the phrase “added element” means an element (or an ionicform of the element) that is added to the semiconductor pattern 300 witha second oxide formation free energy whose absolute value is greaterthan or equal to the absolute value of a first oxide formation freeenergy of a metal element included in the source and drain electrodes210 and 230. In this exemplary embodiment, the added element does notmean simply an element added in isolation to an oxide semiconductor but,rather, refers to an element or ion added to an oxide semiconductor inconsideration of the oxide formation free energy of a metal included inthe source metal electrode 210 and the drain electrode 230. The addedelement may be present in an ionic form or as an oxide.

Table 1 shows oxide formation free energies of various metal elements atabout 100° C.

TABLE 1 Oxide formation free energy Element [kcal/mol] Pd −18.5 Cu −29Se −37.7 Te −47.4 Ni −48.8 Co −49.8 Cs −51 Bi −56.51 Ru −57.26 Ge −61.6Zn −74.8 Fe −86.3 As −89.2 Re −90 Sb −94.8 In −96.4 Mn −107.9 Ga −116.3Sn −119.6 Ba −123.8 Cr −124 Sr −133.4 Mg −134 Ca −142.4 Mo −155 V −165.7W −177.9 Al −186.2 La −201.3 Si −201.4 Ce −201.5 Pr −202.9 Nd −203.1 Nb−207 Ti −209.3 Y −214.4 Sc −214.8 Ta −224.3 Zr −245 Hf −256.7

Referring to Table 1, at a temperature of about 100° C., an oxideformation free energy for formation of indium oxide from indium is about−96.4 kcal mol⁻¹, and the oxide formation free energy for formation oftitanium oxide from titanium is about −209.3 kcal mol⁻¹. That is, anabsolute value of oxide formation free energy of Ti is greater than theabsolute value of oxide formation free energy of In, i.e.,|ΔG_(ox)(Ti)|>|ΔG_(ox)(In)|.

Since the oxide formation free energies for the metals shown in Table 1are negative valued, a large absolute value of oxide formation freeenergy of these metals means that metal oxide formation isthermodynamically favorable. Consequently, Ti forms an oxide morereadily than In.

When an oxide semiconductor of the semiconductor pattern 300 includesIn, and the source and drain electrodes 210 and 230 include Ti, Ti maybe oxidized while an In ion in the oxide semiconductor may be reduced.Therefore, In may be extracted from the oxide semiconductor due to thedifference of absolute values between the oxide formation free energy ofthe In and the oxide formation free energy of Ti. In other words, adeoxidation of In ions in the oxide semiconductor may be coupled tooxidation of Ti included in the source and drain electrodes 210 and 230.

When In or Ga included in the oxide semiconductor is reduced to beextracted, the composition of a channel layer of a TFT may vary,inducing a decrease in charge mobility. In addition, a threshold voltagetemporally varies. Moreover, resistance values of the source electrode210 and the drain electrode 230 may increase due to metal extractionfrom the oxide semiconductor. Thus, the TFT may not function because ofthe metal loss, or the switching element may suffer electricalinstability and decreased reliability.

As described above, extraction of elements, for example, In or Ga fromthe oxide semiconductor may be couples to oxidation of a metal in thesource and drain electrodes 210 and 230. Therefore, prevention ofoxidation of a metal in the source and drain electrodes 210 and 230 thatcontact the semiconductor pattern 300 is a favorable objective.

According to an exemplary embodiment of the present invention, toprevent oxidation of a metal in the source and drain electrodes 210 and230 that contact the semiconductor pattern 300, the semiconductorpattern 300 may include an added element having an absolute value of asecond oxide formation free energy that is greater than or equal to theabsolute value of the first oxide formation free energy for a metal inthe source and drain electrodes 210 and 230. For example, when thesource and drain electrodes 210 and 230 include Ti, the added element ofthe semiconductor pattern 300 may include at least one of Ti, Y, Sc, Ta,Zr, and Hf. These elements have an absolute value of a second oxideformation free energy that is greater than or equal to the absolutevalue of a first oxide formation free energy of Ti.

For an absolute value of a second oxide formation free energy of anadded element in the semiconductor pattern 300 is greater than theabsolute value of a first oxide formation free energy of a metal in thesource and drain electrodes 210 and 230, the added element may be limitor diminish the oxidation of the metal included in the source and drainelectrodes 210 and 230. Accordingly, deoxidation or extraction of an ionin the oxide semiconductor may be diminished.

For example, when the source and drain electrodes 210 and 230 include Tiand the semiconductor pattern 300 includes Ta, the absolute value of asecond oxide formation free energy of Ta, which is about 224.3 kcal atabout 100° C., is greater than the absolute value of a first oxideformation free energy of Ti, which is about 209.3 kcal at about 100° C.Thus, Ta as an added element may decrease the amount of oxidation of theTi. When oxidation of the Ti is restrained, a deoxidation or anextraction of an ion, e.g., In, in the oxide semiconductor may beconcomitantly restrained.

When the source and drain electrodes 210 and 230 include aluminum (Al),the added element of the semiconductor pattern 300 may include at leastone of aluminum (Al), lanthanum (La), silicon (Si), cerium (Ce),praseodymium (Pr), neodymium (Nd), niobium (Nb), titanium (Ti), yttrium(Y), scandium (Sc), tantalum (Ta), zirconium (Zr) and hafnium (Hf)having an absolute value of a second oxide formation free energy that isgreater than or equal to an absolute value of a first oxide formationfree energy of the aluminum (Al). When the source and drain electrodes210 and 230 include aluminum (Al), the added element of thesemiconductor pattern 300 may include at least one of molybdenum (Mo),vanadium (V), tungsten (W), aluminum (Al), lanthanum (La), silicon (Si),cerium (Ce), praseodymium (Pr), neodymium (Nd), niobium (Nb), titanium(Ti), yttrium (Y), scandium (Sc), tantalum (Ta), zirconium (Zr) andhafnium (Hf) having an absolute value of a second oxide formation freeenergy that is greater than or equal to an absolute value of a firstoxide formation free energy of the molybdenum (Mo).

FIG. 3 shows photographs of the decrease in the amount of a metalextracted from an oxide semiconductor corresponding to an increase in anamount of an added element included in the oxide semiconductor. In FIG.3, the source electrode and the drain electrode are represented byreference labels “SE” and “DE,” respectively.

In the photographs, a Si-doped donor was used as the gate electrode, andsilicon oxide (SiOx) having a thickness of about 1 μm was used as thegate insulation layer. In addition, Zn—Sn oxide was used as an oxidesemiconductor forming the channel layer, and titanium (Ti) and platinum(Pt) having a very low reactivity were used as source-drain electrodes.

Tantalum (Ta) as an added element was introduced into the Zn—Sn oxidesemiconductor in a radio frequency (RF)-magnetron sputtering processusing a tantalum oxide (Ta₂O₃) target. In FIG. 3, about 20 watts, about30 watts, about 40 watts, about 50 watts, and about 70 watts of RF powerwas supplied during sputtering to vary the amount of Ta delivered to theZn—Sn oxide semiconductor. As the RF power increases, the amount of Taadded in the Zn—Sn oxide semiconductor increases.

A heat treatment process for the five samples shown in the photographsin FIG. 5 was performed in an air atmosphere at about 350° C. for aboutone hour. The migration of ions in the Zn—Sn oxide semiconductor due totheir deoxidation and extraction was photographed using electronmicroscopy.

In FIG. 3, black spots on the source and the drain electrodes SE and DEindicate that ions in the Zn—Sn oxide semiconductor are reduced andextracted upon oxidation of Ti in the source and drain electrodes SE andDE by the oxide semiconductor. The number of black spots decreases asthe amount of Ta is added to the oxide semiconductor, which occurs withincreasing RF power of the sputter source. That is, increasing theamount of Ta present in the oxide semiconductor via, e.g., sputtering,increases the amount of ions in the oxide semiconductor left oxidized,i.e., non-reduced, and, therefore, not extracted. In other words, whenTa, which has an absolute value of an oxide formation free energygreater than the absolute value of an oxide formation free energy of Znor Sn, was included in the Zn—Sn oxide semiconductor, the deoxidation orextraction of ions included in the oxide semiconductor may berestrained.

FIG. 4 is a graph showing a relationship between RF power duringsputtering and charge carrier mobility in an oxide semiconductor. Thedata in FIG. 4 corresponds to conditions used in the photographs shownin FIG. 3.

In the graph shown in FIG. 4, the horizontal axis indicates the RF powerduring used in the RF-magnetron sputtering process, and the verticalaxis indicates the mobility of a charge carrier within the respectiveoxide semiconductor, which is either Zn—Sn in a 3:1 ratio (closed squaresymbols) or 1:1 ratio (closed circular symbols). Charge mobility is thearea that a carrier traverses per unit time and unit voltage and hasunits of cm²/Vsec.

According to FIG. 4, as the amount of Ta added to the oxidesemiconductor increases, reflected by increasing RF power, the mobilitydecreases. That is, when the amount of the added element, e.g., Ta, inthe oxide semiconductor increases, the mobility within the semiconductordecreases and reduces the electrical characteristics or functions of aTFT.

As described above, a deoxidation of a metal in the semiconductorpattern 300 may be restrained so that a deoxidation or an extraction ofion included in the semiconductor pattern 300 may be restrained.However, when the semiconductor pattern 300 greatly includes the addedelement, the mobility within an oxide semiconductor may be greatlydecreased so that electric characteristics or function of a TFT may bereduced. Thus, an amount of the added element may be smaller than thatof major elements included in the oxide semiconductor of thesemiconductor pattern 300 such as indium (In), gallium (Ga), zinc (Zn),tin (Sn), etc.

However, when an amount of the added element is decreased so as toincrease mobility, the oxidation of a metal in the source and drainelectrodes 210 and 230 may increase. In addition, a deoxidationrestraining effect or an extraction restraining effect of an ionincluded in the oxide semiconductor pattern 300 may decrease. Thus,minimizing the mobility decrease of the oxide semiconductor withoutdecreasing the positive effects of the added element, i.e., restrainingextraction of an ion in the oxide semiconductor pattern 300, isbeneficial.

Hereinafter, a surface of the semiconductor pattern 300 that contactsthe gate insulation layer 150 is called a first surface 301, and asurface of the semiconductor pattern 300 that contacts the source anddrain electrodes 210 and 230 is called a second surface 302. The secondsurface 302 opposes the first surface 301.

Since the added element may play a role in restraining oxidation of ametal in the source and drain electrodes 210 and 230, the added elementmay be distributed at a portion near the second surface 302 of thesemiconductor pattern 300.

Since the first surface 301 of the semiconductor pattern 300 does notcontact the source and drain electrodes 210 and 230, restraining theoxidization of a metal in the source and drain electrodes 210 and 230may not occur even if a portion near the first surface 301 includes theadded element in a relatively small amount. When the portion near thefirst surface 301 includes the added element in a relatively smallamount, a decrease of mobility within the oxide semiconductor pattern300 may be prevented. Accordingly, a distribution of the added elementincluded in the semiconductor pattern 300 may be adjusted so that adecrease of mobility within the oxide semiconductor may be minimized.

According to an exemplary embodiment of the present invention, an amountof the added element included in a portion near the first surface 301 ofthe semiconductor pattern 300 is zero or smaller than an amount of theadded element included in a portion near the second surface 302 of thesemiconductor pattern 300. Hereinafter, the term “an amount of an addedelement is zero” means that the added element is not included in thesemiconductor pattern 300. Further, the added element may not bedistributed uniformly in the semiconductor pattern 300 but may bepresent in higher concentrations near the second surface 302 contactingthe source and drain electrodes 210 and 230 rather than the firstsurface 301 contacting the gate insulation layer 150 within thesemiconductor pattern 300. Moreover, a portion near the first surface301 contacting the gate insulation layer 150 may not include the addedelement.

FIG. 5A and FIG. 5B are graphs showing a relationship between an amountof an added element in an oxide semiconductor and a distance from afirst surface of an oxide semiconductor pattern shown in FIG. 2.

In FIG. 5A and FIG. 5B, the vertical axes indicate a distance from thefirst surface 301 within the semiconductor pattern 300, and thehorizontal axes indicate an amount of Zn in the semiconductor pattern300. The term “d” indicates a thickness of the semiconductor pattern 300or a distance between the first surface 301 and the second surface 302,as shown in FIG. 2. The distance and amount may have an arbitrary unitrepresenting a relative amount.

In an exemplary embodiment, the amount of the added element in thesemiconductor pattern 300 may be gradually decreased near the firstsurface 301 away from the second surface 302, as shown in FIG. 5A.

The amount of the added element in the semiconductor pattern maygradually decrease near the first surface 301 away from the secondsurface 302 through an annealing process. For example, when the addedelement is formed at the second surface 302 of the semiconductor pattern300 through a sputtering process using a target including an oxidematerial of the added element followed by annealing for about tenminutes to about four hours in an air atmosphere or a nitrogenatmosphere in a temperature range of about 200° C. to about 400° C., theadded element at the second surface 302 diffuses in the semiconductorpattern 300 toward the first surface 301. When the annealing time isextremely long, the added element may be uniformly distribute in thesemiconductor pattern 300 so that the added element may not establish aconcentration gradient in the semiconductor pattern 300. Thus, theannealing temperature and time may be adjusted according to the identityof the added element, the material of the oxide semiconductor, and areaction speed of the added element.

In another exemplary embodiment, the amount of the added elementincluded in the semiconductor pattern 300 may be decreased in astep-like fashion with respect to the first and second surfaces 301 and302 as shown in FIG. 5B. For example, when plural sub-semiconductorpatterns including added elements of the different amounts are depositedthereon, a structure in which the amount of the added element isdecreased in a step shape as the sub-semiconductor patterns become nearthe first surface 301 from the second surface 302 may be realized.

A distribution of the added element included in the semiconductorpattern 300 shown in FIG. 5A and FIG. 5B is described as an example.However, the distribution of the added element in the semiconductorpattern 300 is not limited to the profiles shown in FIG. 5A and FIG. 5B.That is, when an amount of the added element included in a portion nearthe first surface 301 is zero or smaller than an amount of the addedelement included in a portion near the second surface 302, thedistribution of the added element may be varied in many ways.

FIG. 6A, FIG. 6B, FIG. 6C and FIG. 6D are graphs showing free energyassociated with oxide formation by reaction of various metals with 1mole of an oxide material of In, Ga, Zn, and Sn.

In the graphs shown in FIG. 6A, FIG. 6B, FIG. 6C and FIG. 6D, a lowerportion of a vertical axis indicates that an absolute value of an oxideformation free energy is large, and an upper portion of the verticalaxis indicates that the absolute value of an oxide formation free energyis small. When the absolute value of the oxide formation free energyassociated with reacting a metal with 1 mole of another metal oxide isgreat, i.e., the lower portion of the vertical axis, a tendency forbecoming an oxide material is great. In other words, when the absolutevalue of the oxide formation free energy is large, a tendency for adeoxidation of a metal ion included in the oxide metal co-reactant islarge.

Referring to FIG. 6A, FIG. 6B, FIG. 6C and FIG. 6D, for indium oxide,gallium oxide, zinc oxide and tin oxide, indium oxide tends to reactwith metals such as Ti, Al, and Ta and non-metals such as Si to bereduced to indium. Moreover, the melting point of In is about 157° C.,which is lower than a process temperature range of a following processsuch as a heat treatment process that may occur in the temperature rangefrom about 250° C. to about 350° C. Thus, when indium ion that may be inan oxide semiconductor reacts with a metal in the source and drainelectrodes 210 and 230 to be reduced, the indium ion may be eluted in aliquid following a process such as a heat treatment process. When an ionincluded on an oxide semiconductor is eluted in a liquid, a reliabilityof a TFT may be decreased or a switching function may be stopped.

A tendency for gallium ion included in an oxide semiconductor to reactwith a metal included in the source and drain electrodes 210 and 230 issmaller than a tendency for indium ion to react with the metal includedin the source and drain electrodes 210 and 230. However, a melting pointof gallium is about 29° C., which is much lower than a temperature rangeof about 250° C. to about 350° C., the process temperature range of afollowing process such as a heat treatment process. Thus, when a galliumion reacts with a metal included in the source and drain electrodes 210and 230 to be deoxidized, the gallium ion may be eluted in a liquid whenthe following process is a heat treatment process.

In contrast, the tendency for zinc oxide to react with a metal such asTi, Al, and or non-metal such as Si to form Zn is very small. Themelting point of Zn is about 420° C., which is much higher than atemperature range of about 250° C. to about 350° C., the processtemperature range of a following process such as a heat treatmentprocess. Thus, although zinc ion may react with a metal included in thesource and drain electrodes 210 and 230 to become deoxidized, the zincion may not be eluted in a liquid when the following process isperformed. Therefore, when the oxide semiconductor of the semiconductorpattern 300 includes zinc ion, regardless of an oxide formation freeenergy of a metal included in the source and drain electrodes 210 and230, oxidation of the metal may be restrained.

Although not shown in FIG. 6A, FIG. 6B, FIG. 6C and FIG. 6D, a tendencyof oxide formation free energy required to form a metal oxide byreacting a metal such as Nd, Nb, Y, and Sc with 1 mole of an oxidematerial of In, Ga, Zn, and Sn is similar with that of metals such asTi, Al, and Ta and non-metals such as Si as shown in FIG. 6A, FIG. 6B,FIG. 6C and FIG. 6D.

FIG. 7 shows photographs of a decrease in the metal amount extractedfrom an oxide semiconductor that corresponds to an increase in theamount of zinc ion included in the oxide semiconductor.

In the photographs of FIG. 7, a Si-doped donor was used as the gateelectrode, and SiOx having a thickness of about 1 μm was used as thegate insulation layer. In addition, Zn—Sn oxide was used as an oxidesemiconductor forming the channel layer, and Ti and Pt having a very lowreactivity towards oxidation were used as source-drain electrodes. Inthis case, three samples in which content ratios of zinc ion and tin ionincluded in the Zn—Sn oxide semiconductor are respectively about 1:1,about 2:1, and about 3:1 were prepared.

A heat treatment process for the three samples was performed in an airatmosphere at about 350° C. for about one hour, and then an amount ofions included in the Zn—Sn oxide semiconductor that have been deoxidizedand extracted were photographed using electron microscopy.

In FIG. 7, black spots on the source and drain electrodes SE and DErepresent ions included in the Zn—Sn oxide semiconductor that aredeoxidized and extracted by oxidizing Ti in the source and drainelectrodes SE and DE with the oxide semiconductor.

Many black spots are present when a content ratio of zinc ions and tinions is about 1:1. However, the number of black spots greatly diminisheswhen a content ratio of zinc ions and tin ions is about 3:1. That is,the amount of ions included in the oxide semiconductor that becomedeoxidized to be extracted decreases, as the amount of zinc ion in theoxide semiconductor increases. In other words, the extraction of ions inthe oxide semiconductor may be restrained as zinc ion is further addedin the oxide semiconductor.

Referring again to the graph of FIG. 4, the mobility corresponding to acontent ratio of zinc ion and tin ion of about 3:1 is smaller than thecorresponding content ratio of zinc ion and tin ion of about 1:1. As theamount of zinc ion added to the oxide semiconductor increases, mobilitydecreases. When the amount of zinc ion added to the oxide semiconductorreaches a relatively large amount, the mobility within the semiconductormay be decreased to such an extent that the electrical characteristicsand functions of a TFT may be reduced.

Since the first surface 301 of the semiconductor pattern 300 does notcontact the source and drain electrodes 210 and 230, an effect ofrestraining oxidation of a metal included in the source and drainelectrodes 210 and 230 may not be reduced even though zinc (Zn) ion isincluded in a relatively small amount in near the first surface 301 ofthe semiconductor pattern 300. When a portion near the first surface 301does not include the added element or includes the added element in arelatively small amount, a decrease of mobility within the oxidesemiconductor pattern 300 may be prevented. Accordingly, a distributionof zinc ion further added in the semiconductor pattern 300 may beadjusted so that a decrease of mobility within the oxide semiconductormay be minimized.

In accord with the above description, regardless of a first oxideformation free energy of a metal included in the source and drainelectrodes 210 and 230, zinc ion may be further added to a portion nearthe second surface 302 contacting the source and drain electrodes 210and 230 so that an oxidation of the metal included in the source anddrain electrodes 210 and 230 may be restrained. When the semiconductorpattern 300 includes an oxide material including an ion of an element ofat least one of In, Ga, Zn, and Sn, an amount of zinc ion included in aportion near the first surface 301 contacting the gate insulation layer150 may be zero or smaller than an amount of zinc ion included in aportion near the second surface 302 contacting the source and drainelectrodes 210 and 230. For example, a portion near the first surface301 may include indium oxide and exclude zinc, and a portion near thesecond surface 302 may include In—Zn oxide or In—Zn—Sn oxide. Moreover,the semiconductor pattern 300 may have a double layer structure in whicha portion near the first surface 301 includes indium oxide, and aportion near the second surface 302 includes In—Zn oxide or In—Zn—Snoxide.

An amount of zinc ion in the semiconductor pattern 300 may be graduallydecreased traversing the semiconductor pattern 300 near the firstsurface 301 away from the second surface 302, as shown in FIG. 5A.Alternatively, an amount of zinc ion in the semiconductor pattern 300may step-wise decrease within the semiconductor pattern 300 near thefirst surface 301 away from the second surface 302, as shown in FIG. 5B.

A distribution of zinc ion included in the semiconductor pattern 300shown in FIG. 5A and FIG. 5B is described as an example. However, thedistributions shown in FIG. 5A and FIG. 5B does not limit distributionsin exemplary embodiments. That is, when an amount of zinc ion in aportion near the first surface 301 is zero or smaller than an amount ofzinc ion in a portion near the second surface 302, the distribution ofzinc ion may vary throughout the semiconductor pattern 300.

In another exemplary embodiment, an amount of the added element havingan absolute value of a second oxide formation free energy that isgreater than or equal to an absolute value of a first oxide formationfree energy of the first metal element may be adjusted in addition toadjustment of a distribution of the zinc ion in the semiconductorpattern 300. For example, amounts of the added element and an amount ofzinc ion in a portion near the first surface 301 may be zero or smallerthan an amount of the added element and a content of the zinc ion in aportion near the second surface 302. Thus, when a distribution of thezinc ion is adjusted, there may also be present an increased amount ofthe added element to minimize a decrease of mobility within the oxidesemiconductor. In addition, an oxidation of a metal in the source anddrain electrodes 210 and 230 reacting with the oxide semiconductor maybe restrained.

FIG. 8 is a cross-sectional view of an oxide semiconductor TFT accordingto another exemplary embodiment of the present invention.

The TFT of FIG. 8 is substantially the same as the TFT show in FIG. 1and FIG. 2 except that the semiconductor pattern 300 has a double layerstructure. Thus, the same reference numerals will be used to refer tothe same or like parts as those described in FIG. 1 and FIG. 2, andrepeated explanation is omitted or simplified.

Referring to FIG. 8, the TFT includes a gate electrode 110, a gateinsulation layer 150, a semiconductor pattern 300, a source electrode210, and a drain electrode 230.

The semiconductor pattern 300 has a double layer structure in which afirst sub-semiconductor pattern 310 and a second sub-semiconductorpattern 320 are disposed. The first semiconductor pattern 310 has afirst surface 301 contacting the gate insulation layer 150, and thesecond sub-semiconductor pattern 320 has a second surface 302 contactingthe source and drain electrodes 210 and 230. That is, the firstsub-semiconductor pattern 310 is disposed to contact the gate insulationlayer 150, and the second sub-semiconductor pattern 320 is disposed tocontact with the source and drain electrodes 210 and 230.

According to an exemplary embodiment of the present invention, toprevent an oxidation of a metal in the source and drain electrodes 210and 230 contacting the semiconductor pattern 300, the secondsub-semiconductor pattern 320, which contacts the source electrode 210and 230, may include an added element having an absolute value of asecond oxide formation free energy that is greater than or equal to anabsolute value of the first oxide formation free energy of the metal inthe source and drain electrodes 210 and 230. For example, when thesource and drain electrodes 210 and 230 include Ti, the added element ofthe second sub-semiconductor pattern 320 may include at least one of Ti,Y, Sc, Ta, Zr, and Hf having an absolute value of a second oxideformation free energy that is greater than or equal to an absolute valueof a first oxide formation free energy of the Ti.

As described above, the added element, which has an absolute value of asecond oxide formation free energy that is greater than or equal to anabsolute value of the first oxide formation free energy of a metalincluded in the source and drain electrodes 210 and 230, is added in thesecond sub-semiconductor pattern 320 so that an oxidation of a metalincluded in the source and drain electrodes 210 and 230 may berestrained. An extraction due to a deoxidation of an ion such as indium(In) or gallium (Ga) included in the oxide semiconductor is caused by anoxidation of a metal included in the source and drain electrodes 210 and230. Thus, when the added element restrains an oxidation of a metalincluded in the source and drain electrodes 210 and 230, a deoxidationor an extraction of an ion included in the oxide semiconductor may berestrained. A principle of restraining an oxidation of a metal includedin the source and drain electrodes 210 and 230 by inclusion of the addedelement and restraining an extraction of an ion in the oxidesemiconductor is described above, and repetitive details are omitted orsimplified.

When the oxide semiconductor includes a large quantity of the addedelement, the mobility within the oxide semiconductor may be greatlydecreased so that electrical characteristics or functions of a TFT maybe reduced. Thus, an amount of the added element included in the firstsub-semiconductor pattern 310 spaced apart from the source and drainelectrodes 210 and 230 may be zero or smaller than an amount of theadded element included in the second sub-semiconductor pattern 320.

Since the first sub-semiconductor pattern 310 does not contact thesource and drain electrodes 210 and 230, an effect of restraining anoxidation of a metal included in the source and drain electrodes 210 and230 is not reduced even though the first sub-semiconductor pattern 310may not include the added element or may include the added element in arelatively small amount. That is, when the first sub-semiconductorpattern 310 does not include the added element or does include the addedelement but in a relatively small amount, a decrease of mobility withinthe oxide semiconductor may be prevented.

As described above, the semiconductor pattern 300 is divided into thefirst sub-semiconductor pattern 310 spaced apart from the source anddrain electrodes 210 and 230 and the second sub-semiconductor pattern320 contacting the source and drain electrodes 210 and 230, and adistribution of the added element in the second sub-semiconductorpattern 320 may be adjusted so that a decrease of mobility within theoxide semiconductor may be minimized.

When the two sub-semiconductor patterns 310 and 320 having differentamounts of the added element are deposited to form the semiconductorpattern 300, even though the added element may be uniformly distributedwithin each of the sub-semiconductor patterns 310 and 320, a decrease ofmobility within the oxide semiconductor may be minimized. In addition,restraining a deoxidation of an ion included in the oxide semiconductormay be achieved. That is, in a manufacturing process of thesub-semiconductor patterns 310 and 320, diffusion of the added elementis not required to adjust a distribution of the added element, which maysimplify the manufacturing process of the semiconductor pattern 300.

In another exemplary embodiment of the present invention, regardless ofa first oxide formation free energy of a metal included in the sourceand drain electrodes 210 and 230, zinc ion may be further added to thesecond semiconductor pattern 320 to restrain oxidation of the metal inthe source and drain electrodes 210 and 230. Restraining oxidation of ametal in the source and drain electrodes 210 and 230 by adding zinc ionto the oxide semiconductor and restraining extraction of an ion in theoxide semiconductor is described above, and repetitive explanation isomitted or simplified.

In this case, an amount of zinc ion in the first sub-semiconductorpattern 310 may be zero or smaller than an amount of zinc ion in thesecond sub-semiconductor pattern 320. For example, the firstsub-semiconductor pattern 310 may include indium oxide in which Zn isnot included, and the second sub-semiconductor pattern 320 may includeIn—Zn oxide or In—Zn—Sn oxide. Alternatively, the firstsub-semiconductor pattern 310 may include indium oxide in which Zn isnot included, and the second sub-semiconductor pattern 320 may includeZn—Sn oxide.

As described above, the semiconductor pattern 300 is layered into thefirst sub-semiconductor pattern 310 and the second sub-semiconductorpattern 320, and distributions of zinc ion respectively in the first andsecond sub-semiconductor patterns 310 and 320 are individually adjustedso that a decrease of mobility within the oxide semiconductor may beminimized.

In another exemplary embodiment, not only may the distribution amount ofthe zinc ion be adjusted but adjustment may be made of an amount of theadded element having an absolute value of a second oxide formation freeenergy that is greater than or equal to an absolute value of a firstoxide formation free energy of the metal element in the source and drainelectrodes 210 and 230. For example, an amount of the added element andan amount of zinc ion in the first sub-semiconductor pattern 310 may bezero or smaller than an amount of the added element and an amount of thezinc ion in the second sub-semiconductor pattern 320. Thus, when adistribution of the zinc ion is adjusted together with a distribution ofthe added element, a decrease of mobility within the oxide semiconductormay be minimized. In addition, an oxidation of a metal included in thesource and drain electrodes 210 and 230 caused by reacting with theoxide semiconductor may be restrained.

FIG. 9 is a cross-sectional view of an oxide semiconductor TFT accordingto another exemplary embodiment of the present invention.

The TFT of FIG. 9 is similar to the TFT of FIG. 1, FIG. 2, and FIG. 8except that the semiconductor pattern 300 has at least a triple layerstructure. Thus, the same reference numerals will be used to refer tothe same or like parts as those described in FIG. 1, FIG. 2, and FIG. 8,and repetitive descriptions are omitted or simplified.

Referring to FIG. 9, the TFT includes a gate electrode 110, a gateinsulation layer 150, a semiconductor pattern 300, a source electrode210 and a drain electrode 230.

The semiconductor pattern 300 has a triple layer structure in which afirst sub-semiconductor pattern 310, a second sub-semiconductor pattern320, and a third sub-semiconductor pattern 330 are disposed.

The first sub-semiconductor pattern 310 has a first surface 301contacting the gate insulation layer 150, and the thirdsub-semiconductor pattern 330 has a second surface 302 contacting thesource and drain electrodes 210 and 230. That is, the firstsub-semiconductor pattern 310 is disposed to contact the gate insulationlayer 150, and the third sub-semiconductor pattern 330 is disposed tocontact the source and drain electrodes 210 and 230. The secondsub-semiconductor pattern 320 is disposed between the firstsub-semiconductor pattern 310 and the third sub-semiconductor pattern330.

According to an exemplary embodiment of the present invention, in orderto prevent oxidation of a metal in the source and drain electrodes 210and 230 that contact the semiconductor pattern 300, the thirdsub-semiconductor pattern 330, which contacts with the source electrode210 and 230, may include an added element having an absolute value of asecond oxide formation free energy that is greater than or equal to anabsolute value of the first oxide formation free energy of the metal inthe source and the drain electrodes 210 and 230. The added element maybe added to the third sub-semiconductor pattern 330 so that oxidation ofthe metal included in the source and drain electrodes 210 and 230 may berestrained. Moreover, when oxidation of the metal included in the sourceand drain electrodes 210 and 230 is restrained by the added element, anextraction of an ion in the oxide semiconductor may be restrained.Restraining oxidation of a metal included in the source and drainelectrodes 210 and 230 by the added element and restraining anextraction of an ion in the oxide semiconductor is described above, andrepetitive explanations are omitted or simplified.

When the oxide semiconductor includes a large amount of the addedelement, the mobility within the oxide semiconductor may be greatlydecreased so that electrical characteristics or function of a TFT may bereduced. Thus, an amount of the added element in the secondsub-semiconductor pattern 320 spaced apart from the source and drainelectrodes 210 and 230 may be zero or smaller than an amount of theadded element included in the third sub-semiconductor pattern 330.Moreover, an amount of the added element included in the firstsub-semiconductor pattern 310 spaced apart from the source and drainelectrodes 210 and 230 may be zero or smaller than an amount of theadded element included in the second and third sub-semiconductorpatterns 320 and 330. In other words, the semiconductor pattern 300 mayhave three sub-semiconductor patterns 310, 320 and 330, and an amount ofthe added element, which is included in a sub-semiconductor pattern,i.e., the first sub-semiconductor pattern 310, disposed near the firstsurface 301 contacting the gate insulation layer 150, may be smallerthan an amount of the added element included in anothersub-semiconductor pattern, e.g., the third sub-semiconductor pattern330, near the second surface 302.

Since the first sub-semiconductor pattern 310 and the secondsub-semiconductor pattern 320 do not contact the source and drainelectrodes 210 and 230, restraining oxidation of the metal in the sourceand drain electrodes 210 and 230 may not decrease even though the firstsub-semiconductor pattern 310 or the second sub-semiconductor pattern320 may include the added element in a relatively small amount.

For example, an amount of the added element may be zero in asub-semiconductor pattern, e.g., the first sub-semiconductor pattern310, disposed near the first surface 301 contacting the gate insulationlayer 150.

In another exemplary embodiment of the present invention, regardless ofa first oxide formation free energy of a metal included in the sourceand drain electrodes 210 and 230, zinc ion may be added to the thirdsemiconductor pattern 330 contacting the source and drain electrodes 210and 230 so that oxidation of the metal included in the source and drainelectrodes 210 and 230 may be restrained. Restraining oxidation of ametal included in the source and drain electrodes 210 and 230 by addingzinc ion to the oxide semiconductor and restraining an extraction of anion in the oxide semiconductor is described above, and repetitiveexplanations are omitted or simplified.

In this case, an amount of zinc ion in the first sub-semiconductorpattern 310 may be zero or smaller than an amount of zinc ion in thesecond sub-semiconductor pattern 320. Accordingly, the semiconductorpattern 300 may be divided into plural sub-semiconductor patterns 310,320 and 330, and distributions of zinc ion in the pluralsub-semiconductor patterns 310, 320 and 330 may be separately adjustedso that a decrease of mobility within the oxide semiconductor may beminimized.

In FIG. 9, three sub-semiconductor patterns 310, 320 and 330 are shown;however, the semiconductor pattern is not limited to just this exemplaryembodiment. For example, when the semiconductor pattern 300 has amulti-layer structure with four sub-semiconductor patterns or fivesub-semiconductor patterns, effects by the multi-layer structure on therestraints of oxidation and extraction discussed above may besubstantially similar to the exemplary embodiment shown in FIG. 9.

In another exemplary embodiment, an amount of the added element havingan absolute value of a second oxide formation free energy that isgreater than or equal to an absolute value of a first oxide formationfree energy of the metal element in the source and drain electrodes 210and 230 may be adjusted together with a distribution of the zinc ion.For example, an amount of the added element and an amount of zinc ion ina sub-semiconductor pattern, e.g., the first sub-semiconductor pattern310, disposed near the first surface 301 may be smaller than an amountof the added element and an amount of zinc ion in a sub-semiconductorpattern, e.g., the third sub-semiconductor pattern 330, disposed nearthe second surface 302. Thus, when a content distribution of zinc ion isadjusted with a distribution of the added element, a decrease ofmobility within the oxide semiconductor may be minimized. In addition,an oxidation of a metal in the source and drain electrodes 210 and 230caused by reaction with the oxide semiconductor may be restrained.

FIG. 10 is a cross-sectional view of an oxide semiconductor TFTaccording to another exemplary embodiment of the present invention.

The TFT of FIG. 10 is substantially similar to the TFT of FIG. 8 exceptthat the semiconductor pattern 300 is only formed on a portion of thegate electrode 110. Thus, the same reference numerals will be used torefer to the same or like parts as those described in FIG. 8, andrepetitive explanations are omitted or simplified.

Referring to FIG. 10, the TFT includes a gate electrode 110, a gateinsulation layer 150, a semiconductor pattern 300, a source electrode210, and a drain electrode 230. The semiconductor pattern 300 has adouble layer structure in which a first sub-semiconductor pattern 310and a second sub-semiconductor pattern 320 are disposed. The firstsub-semiconductor pattern 310 has a first surface 301 contacting thegate insulation layer 150, and the second sub-semiconductor pattern 320has a second surface 302 contacting the source and drain electrodes 210and 230. That is, the first sub-semiconductor pattern 310 is disposed tocontact the gate insulation layer 150, and the second sub-semiconductorpattern 320 is disposed to contact the source and drain electrodes 210and 230.

The semiconductor pattern 300 of the TFT of FIG. 8 is formed below thesource and drain electrodes 210 and 230; however, the semiconductorpattern 300 of the TFT of FIG. 10 is formed only on a portion of thegate electrode 110. In a case of the TFT of FIG. 8, since the sourceelectrode 210, the drain electrode 230 and the semiconductor pattern 300may be patterned through the same mask, the semiconductor pattern 300 isformed below the source and drain electrodes 210 and 230. However, sincethe semiconductor pattern 300 of the TFT of FIG. 10 is patterned througha different mask than used for patterning the source and drainelectrodes 210 and 230, the semiconductor pattern 300 is formed onlywhere the gate electrode 110 is formed.

Although a pattern structure of the semiconductor pattern 300 of the TFTshown in FIG. 10 is different from a pattern structure of thesemiconductor pattern 300 of the TFT shown in FIG. 8, principlesdiscussed above are applicable to each of the semiconductor patterns.

That is, to prevent oxidation of a metal in the source and drainelectrodes 210 and 230 contacting the semiconductor pattern 300, thesecond sub-semiconductor pattern 320, which contacts the source anddrain electrodes 210 and 230, may include an added element having anabsolute value of a second oxide formation free energy that is greaterthan or equal to an absolute value of the first oxide formation freeenergy of the metal in the source and drain electrodes 210 and 230.Restraining oxidation of the metal in the source and drain electrodes210 and 230 by the added element and restraining an extraction of an ionin the oxide semiconductor is described above, and repetitiveexplanations are omitted or simplified.

In order to minimize a decrease of charge mobility, an amount of theadded element in the first sub-semiconductor pattern 310 that does notcontact the source and drain electrodes 210 and 230 may be zero orsmaller than an amount of the added element in the secondsub-semiconductor pattern 320.

Moreover, regardless of a first oxide formation free energy of a metalincluded in the source and drain electrodes 210 and 230, zinc ion may beadded to the second sub-semiconductor pattern 320 contacting the sourceand drain electrodes 210 and 230 so that oxidation of the metal in thesource and drain electrodes 210 and 230 may be restrained. Restrainingoxidation of the metal included in the source and drain electrodes 210and 230 by adding zinc ion to the oxide semiconductor and restraining anextraction of an ion in the oxide semiconductor is described above, andrepetitive explanations are omitted or simplified.

Moreover, an amount of the added element having an absolute value of asecond oxide formation free energy that is greater than or equal to anabsolute value of a first oxide formation free energy of the metalelement in the source and drain electrodes 210 and 230 may be adjustedtogether with a distribution of zinc ion. For example, an amount of theadded element and an amount of zinc ion in the first sub-semiconductorpattern 310 may be zero or smaller than an amount of the added elementand an amount of zinc ion in the second sub-semiconductor pattern 320.Thus, when a distribution of zinc ion is adjusted together with adistribution of the added element, a decrease of mobility within theoxide semiconductor may be minimized. In addition, oxidation of themetal included in the source and drain electrodes 210 and 230 resultingfrom reaction with the oxide semiconductor may be restrained.

FIG. 11 is a cross-sectional view of an oxide semiconductor TFTaccording to another exemplary embodiment of the present invention.

The TFT of FIG. 11 is substantially similar to the TFT of FIG. 8 exceptthat the gate electrode 110 is disposed on the semiconductor pattern300, and the source and drain electrodes 210 and 230 are disposed underthe semiconductor pattern 300. The same reference numerals will be usedto refer to the same or like parts as those described in FIG. 8, andredundant explanations are omitted or simplified.

Referring to FIG. 11, the TFT includes a gate electrode 110, a gateinsulation layer 150, a semiconductor pattern 300, a source electrode210, and a drain electrode 230. The semiconductor pattern 300 has adouble layer structure in which a first sub-semiconductor pattern 310and a second sub-semiconductor pattern 320 are disposed. The firstsemiconductor pattern 310 has a first surface 301 contacting the gateinsulation layer 150, and the second sub-semiconductor pattern 320 has asecond surface 302 contacting the source and drain electrodes 210 and230. That is, the first sub-semiconductor pattern 310 is disposed tocontact the gate insulation layer 150, and the second sub-semiconductorpattern 320 is disposed to contact the source and drain electrodes 210and 230.

The TFT shown in FIG. 11 has a top gate structure in which the gateelectrode 110 is disposed on the semiconductor pattern 300, and thesource and drain electrodes 210 and 230 are disposed under thesemiconductor pattern 300.

Although a structure of the TFT shown in FIG. 11 is different from astructure of the TFT shown in FIG. 8, principle of oxidation restraintand extraction restraint discussed above may be substantially applicableto each of the TFTs.

That is, to prevent oxidation of a metal in the source and drainelectrodes 210 and 230 contacting the semiconductor pattern 300, thesecond sub-semiconductor pattern 320, which contacts the sourceelectrode 210 and 230, may include an added element having an absolutevalue of a second oxide formation free energy that is greater than orequal to an absolute value of the first oxide formation free energy ofthe metal in the source and drain electrodes 210 and 230. Restrainingoxidation of the metal included in the source and drain electrodes 210and 230 by the added element and restraining an extraction of an ion inthe oxide semiconductor is described above, and repetitive explanationsare omitted or simplified.

To minimize the decrease in charge mobility of the semiconductor pattern300, an amount of the added element in the first sub-semiconductorpattern 310 that does not contact the source and drain electrodes 210and 230 may be zero or smaller than an amount of the added element inthe second sub-semiconductor pattern 320.

Moreover, regardless of a first oxide formation free energy of a metalin the source and drain electrodes 210 and 230, zinc ion may be added tothe second sub-semiconductor pattern 320 contacting the source and drainelectrodes 210 and 230 so that oxidation of the metal in the source anddrain electrodes 210 and 230 may be restrained. Restraining oxidation ofthe metal in the source and drain electrodes 210 and 230 by adding zincion to the oxide semiconductor and restraining an extraction of an ionin the oxide semiconductor is described above, and repetitiveexplanations are omitted or simplified.

Moreover, an amount of the added element having an absolute value of asecond oxide formation free energy that is greater than or equal to anabsolute value of a first oxide formation free energy of the metalelement in the source and drain electrodes 210 and 230 may be adjustedtogether with a distribution of zinc ion. For example, an amount of theadded element and an amount of zinc ion in the first sub-semiconductorpattern 310 may be zero or smaller than an amount of the added elementand an amount of zinc ion in the second sub-semiconductor pattern 320.Thus, when a distribution of zinc ion is adjusted with a distribution ofthe added element, a decrease of mobility within the oxide semiconductormay be minimized. In addition, oxidation of the metal included in thesource and drain electrodes 210 and 230 resulting from reaction with theoxide semiconductor may be restrained.

Thus, in TFT's according to various exemplary embodiments of the presentinvention, a decrease of mobility within the oxide semiconductor may beminimized, and oxidation of a metal included in the source and drainelectrodes resulting from reaction with the oxide semiconductor may berestrained. Moreover, an effect of restraining a deoxidation of an ionincluded in the oxide semiconductor may be achieved.

The TFT disposed on a pixel area of an array substrate is shown in FIG.1; however, a usage of the TFT according to the present invention is notlimited to that exemplary embodiment. For example, the TFT may beadopted as a TFT used to a scan driving circuit disposed at a peripheralof a pixel area or a shift register disposed at the peripheral of thepixel area in addition to other uses.

The foregoing is illustrative of the present invention and is not to beconstrued as limiting thereof. Although a few example embodiments of thepresent invention have been described, those skilled in the art willreadily appreciate that many modifications are possible in the exemplaryembodiments without departing from the novel teachings and advantages ofthe present invention.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. An oxide semiconductor thin-film transistor (TFT), comprising: a gateelectrode disposed on a substrate; a source electrode and a drainelectrode both comprising a first metal element with a first oxideformation free energy; a gate insulation layer insulating the gateelectrode from the source electrode and the drain electrode; and anoxide semiconductor pattern comprising: a first surface contacting thegate insulation layer; a second surface contacting the source electrodeand the drain electrode, the second surface disposed opposite the firstsurface; and an added element with a second oxide formation free energy,the absolute value of the second oxide formation free energy beinggreater than or equal to the absolute value of the first oxide formationfree energy, wherein, in the oxide semiconductor pattern, an amount ofthe added element in a portion proximate to the first surface is zero orsmaller than an amount of the added element in a portion proximate tothe second surface.
 2. The oxide semiconductor TFT of claim 1, whereinthe first metal element comprises titanium (Ti), and the added elementcomprises at least one of titanium (Ti), yttrium (Y), scandium (Sc),tantalum (Ta), zirconium (Zr), and hafnium (Hf).
 3. The oxidesemiconductor TFT of claim 1, wherein the first metal element comprisesaluminum (Al), and the added element comprises at least one of aluminum(Al), lanthanum (La), silicon (Si), cerium (Ce), praseodymium (Pr),neodymium (Nd), niobium (Nb), titanium (Ti), yttrium (Y), scandium (Sc),tantalum (Ta), zirconium (Zr), and hafnium (Hf).
 4. The oxidesemiconductor TFT of claim 1, wherein the first metal element comprisesmolybdenum (Mo), and the added element comprises at least one ofmolybdenum (Mo), vanadium (V), tungsten (W), aluminum (Al), lanthanum(La), silicon (Si), cerium (Ce), praseodymium (Pr), neodymium (Nd),niobium (Nb), titanium (Ti), yttrium (Y), scandium (Sc), tantalum (Ta),zirconium (Zr), and hafnium (Hf).
 5. The oxide semiconductor TFT ofclaim 1, wherein the oxide semiconductor pattern comprises a firstsub-semiconductor pattern contacting the gate insulation layer and asecond sub-semiconductor pattern contacting the source electrode and thedrain electrode, and an amount of the added element in the firstsub-semiconductor pattern is zero or smaller than an amount of the addedelement in the second sub-semiconductor pattern.
 6. The oxidesemiconductor TFT of claim 5, wherein the first sub-semiconductorpattern and the second sub-semiconductor pattern both comprise an oxidematerial comprising an ion of at least one of indium (In), gallium (Ga),zinc (Zn), and tin (Sn), the second sub-semiconductor pattern comprisesZn ion, and an amount of Zn ion in the first sub-semiconductor patternis zero or smaller than an amount of Zn ion in the secondsub-semiconductor pattern.
 7. The oxide semiconductor TFT of claim 1,wherein the oxide semiconductor pattern comprises at least threesub-semiconductor patterns, and an amount of the added element in thesub-semiconductor pattern proximally disposed to the first surface issmaller than an amount of the added element in the sub-semiconductorpattern proximally disposed to the second surface.
 8. The oxidesemiconductor TFT of claim 7, wherein the amount of the added element inthe sub-semiconductor pattern proximally disposed to the first surfaceis substantially zero.
 9. The oxide semiconductor TFT of claim 7,wherein the at least three sub-semiconductor patterns each comprise anoxide material comprising an ion of at least one of indium (In), gallium(Ga), zinc (Zn) and tin (Sn), and an amount of Zn ion in thesub-semiconductor pattern proximally disposed to the first surface issmaller than an amount of Zn ion in the sub-semiconductor patternproximally disposed to the second surface.
 10. The oxide semiconductorTFT of claim 9, wherein the amount of Zn ion in the sub-semiconductorpattern proximally disposed to the first surface is substantially zero.11. The oxide semiconductor TFT of claim 1, wherein an amount of theadded element gradually decreases from the second surface to the firstsurface, the largest amount being in a portion of the oxidesemiconductor pattern proximal to the second surface.
 12. The oxidesemiconductor TFT of claim 1, wherein the oxide semiconductor patterncomprises an oxide material comprising an ion of at least one of indium(In), gallium (Ga), zinc (Zn) and tin (Sn), and an amount of Zn ion inthe oxide semiconductor pattern gradually decreases from the secondsurface to the first surface, the largest amount being in a portion ofthe oxide semiconductor pattern proximal to the second surface.
 13. Anoxide semiconductor thin-film transistor (TFT), comprising: a gateelectrode disposed on a substrate; a source electrode and a drainelectrode both comprising a first metal element; a gate insulation layerinsulating the gate electrode from the source electrode and the drainelectrode; and an oxide semiconductor pattern comprising: an oxidematerial comprising an ion of at least one of indium (In), gallium (Ga),zinc (Zn), and tin (Sn); a first surface contacting the gate insulationlayer; and a second surface contacting the source electrode and thedrain electrode, the second surface disposed opposite the first surface,wherein, in the oxide semiconductor pattern, a portion proximal to thesecond surface further comprises Zn ion, and an amount of Zn ion in aportion proximal to the first surface is zero or smaller than the amountof Zn ion in the portion proximal to the second surface.
 14. The oxidesemiconductor TFT of claim 13, wherein the oxide semiconductor patternfurther comprises a first sub-semiconductor pattern contacting the gateinsulation layer and a second sub-semiconductor pattern contacting thesource electrode and the drain electrode, the second sub-semiconductorpattern comprises Zn ion, and an amount of Zn ion in the firstsub-semiconductor pattern is zero or smaller than an amount of Zn ion inthe second sub-semiconductor pattern.
 15. The oxide semiconductor TFT ofclaim 13, wherein the oxide semiconductor pattern further comprises atleast three sub-semiconductor patterns, and an amount of Zn ion in thesub-semiconductor pattern proximally disposed to the first surface issmaller than an amount of Zn ion in the sub-semiconductor patternproximally disposed to the second surface.
 16. The oxide semiconductorTFT of claim 15, wherein the amount of Zn ion in the sub-semiconductorpattern proximally disposed to the first surface is substantially zero.17. The oxide semiconductor TFT of claim 13, wherein the amount of Znion in the oxide semiconductor pattern gradually decreases from thesecond surface to the first surface, the largest amount being in aportion of the oxide semiconductor pattern proximal to the secondsurface.
 18. The oxide semiconductor TFT of claim 13, wherein the firstmetal element has a first oxide formation free energy, and the oxidesemiconductor pattern further comprises an added element with a secondoxide formation free energy, the absolute value of the second oxideformation free energy is greater than or equal to the absolute value ofthe first oxide formation free energy.
 19. The oxide semiconductor TFTof claim 18, wherein an amount of the added element in a portionproximal to the first surface is zero or smaller than an amount of theadded element in a portion proximal to the second surface.
 20. The oxidesemiconductor TFT of claim 13, wherein the first metal element comprisesat least one of titanium (Ti), aluminum (Al), tantalum (Ta), silicon(Si), neodymium (Nd), niobium (Nb), yttrium (Y) and scandium (Sc).